1. Field of the Invention
The invention relates to a measurement apparatus and process for determining the position of an object relative to a reference surface, with at least one measurement nozzle, at least one reference nozzle, and at least one measurement device. The outlet opening(s) of the measurement nozzle(s) are located in the reference surface, and by way of a compressed air source air can be made available with a given feed pressure whereby the measurement device acquires one state variable or at least one of three state variables such as flow rate, pressure and speed of the air upstream of the measurement nozzle or measurement nozzles. In essence, the invention relates to pneumatic length measurement or pneumatic distance measurement or determination, the position of one object relative to a reference surface being regarded as the distance to be measured or determined.
2. Description of Related Art
Regarding the principles of pneumatic length measurement and the terminology used therein, the possible applications of pneumatic length measurement and the advantages associated with pneumatic length measurement, reference is made first of all to DIN 2271 xe2x80x9cPneumatic length measurementxe2x80x9d, Part 1 xe2x80x9cPrinciples, Processes,xe2x80x9d September 1976, DIN 2271 xe2x80x9cPneumatic length measurement,xe2x80x9d Part 2 xe2x80x9cConstruction features of devices for the high pressure range,xe2x80x9d April 1976, DIN 2271-3 xe2x80x9cPneumatic length measurement,xe2x80x9d Part 3 xe2x80x9cFeatures of devices for the high pressure range, Requirements, Testing,xe2x80x9d February 2000, and DIN 2271 xe2x80x9cPneumatic length measurement,xe2x80x9d Part 4 xe2x80x9cGeneral data for application and examples,xe2x80x9d November 1977, to literature references xe2x80x9cPRODUCTION ENGINEERING Ixe2x80x9d published by Cert. Teacher. Alfred Reichard, professor in Pforzheim 7, revised edition, pages 46 to 50, and xe2x80x9cNew Innovations in Air Gauging and Control,xe2x80x9d xe2x80x9cTECHNOLOGY NEWS INTERNATIONAL,xe2x80x9d November/December 1999, to the brochure xe2x80x9cSYSTEM FOR PNEUMATIC-ELECTRICAL CONTROL AND MEASUREMENTxe2x80x9d of Mawomatic Mayer Wonisch Mietzel GmbH, 59757 Arnsberg, and to the German patent disclosure document 42 32 630, German patent disclosure document 43 44 264, German patent disclosure document 197 34 374, German patent disclosure document 199 44 163, German Patent utility model 200 04 783, European Patent No. 0 380 967, European Patent No. 0 794 035 and U.S. Pat. No. 3,863,493.
In the pneumatic length measurement under consideration there are on the one hand series circuits, on the other networks, in the networks, those without bridges and those with bridges (compare DIN 2271, Part 1, Section 6 xe2x80x9cCircuitxe2x80x9d, 6.1 xe2x80x9cSeries circuitxe2x80x9d and 6.2 xe2x80x9cNetworkxe2x80x9d, 6.2.1 xe2x80x9cNetwork without bridgexe2x80x9d and 6.2.2 xe2x80x9cNetwork with bridgexe2x80x9d).
It is noted that the expression xe2x80x9cmeasured value transducerxe2x80x9d is also used for the expression xe2x80x9cmeasurement nozzlexe2x80x9d and the expression xe2x80x9czero setterxe2x80x9d is also used for the expression xe2x80x9creference nozzlexe2x80x9d (compare DIN 2271, part 2, Section 3 xe2x80x9cDisplay Devicesxe2x80x9d).
In accordance with the present invention, a measurement apparatus and process is provided, and by way of a compressed air source air with a specific feed pressure can be made available. Generally, the compressed air source is not a component of the measurement means under consideration, but is rather connected to an external compressed air source. This external compressed air source can be made such that it makes available air with the desired feed pressure relatively accurately and constantly over time. However, in the measurement apparatus in accordance with the invention, a filter is connected downstream of the compressed air connection, and also a pressure regulator (compare DIN 2271, Part 1, Section 3 xe2x80x9cMeasurement processxe2x80x9d and Section 6 xe2x80x9cCircuitxe2x80x9d) which makes available compressed air with a certain feed pressure relatively constantly over time within the measurement mechanism.
In pneumatic length measurement, with reference to the feed pressure, it is distinguished between high pressure and low pressure (compare DIN 2271, Part 1, Section 5 xe2x80x9cPressure rangesxe2x80x9d), high pressure being defined here as a feed pressure of xe2x89xa70.5 bar; low pressure being defined here as a feed pressure of xe2x89xa60.1 bar. Preferably, high pressure is used in the above explained sense.
In accordance with the invention, the measurement apparatus includes a measurement device that acquires one state variable or at least one of the three state variables such as flow rate, pressure and speed of the air upstream of at least one measurement nozzle. Essentially, in pneumatic length measurement, the dimension change, the change of the distance of the object to the reference surface, i.e., the gap change, is converted into a change in the flow rate and is detected (compare DIN 2271, Part 1, Section 3 xe2x80x9cMeasurement processxe2x80x9d). Here we distinguish between (a) the process of measuring the flow rate in which the change in the flow rate is directly detected, (b) the pressure measurement process in which the change of flow rate is converted via a preliminary nozzle into a pressure change which is then detected, and (c) the speed measurement process in which the change of the flow rate is converted by a suitable throttle (Venturi nozzle) into a speed change which yields a pressure difference which is detected.
While therefore for the application of the flow rate measurement process only one measurement nozzle and one measurement device which detects the change of flow rate are required, for the application of the pressure measurement process and for the application of the speed measurement process, at least one additional part at a time is required, specifically, in the pressure measurement process at least one preliminary nozzle and in the speed measurement process, at least one suitable throttle (Venturi nozzle).
Several measurement nozzles can be used for the measurement means under consideration and in the process under consideration (parallel circuit (summation circuit) of measurement nozzles, compare DIN 2271, Part 1, Subsection 6.2.3, with FIG. 8). Only one measurement nozzle is ever assumed below; likewise embodiments will always be encompassed which work with several measurement nozzles, in which therefore a parallel circuit (summation circuit) of measurement nozzles is implemented.
Within the framework of the invention, a measurement device can be used which detects one of the three state variables: flow rate, pressure and speed of the air in front of the measurement nozzle. However, several measurement devices can also be implemented which all detect either the same state variable, thus, the flow rate, the pressure, or the speed, or detect different state variables. Therefore, a first measurement device detects the state variable flow rate and the second measurement device the state variable pressure, or the first measurement device the state variable flow rate and the second measurement device the state variable speed, or the first measurement device the state variable pressure and the second measurement device the state variable speed, or the first measurement device the state variable flow rate, the second measurement device the state variable pressure and the third measurement device the state variable speed. It is always assumed below that there is only one measurement device which detects one of the three state variables flow rate, pressure and speed; likewise embodiments will always be encompassed which work with several measurement devices.
While the aforementioned DIN 2271 is referred as xe2x80x9cPneumatic length measurement,xe2x80x9d the focus is on determination of the position of an object relative to a reference surface and it is stated that measurement device xe2x80x9cdetectsxe2x80x9d one state variable or the measurement devices xe2x80x9cdetectxe2x80x9d at least one of the three state variables: flow rate, pressure and speed of the air in front of the measurement nozzle or the measurement nozzles. This difference in terminology has the following justification.
xe2x80x9cMeasurementxe2x80x9d could be defined as proportional detection of the distance of an object relative to the reference surface. But the invention is not limited to this proportional detection, which could also be called xe2x80x9cmeasurement in the narrower sense.xe2x80x9d Rather, within the framework of the invention it can be sufficient if the position of the object relative to a reference surface is determined only to the extent that information can be obtained about whether the distance of the object to the reference surface is greater than or less than a given boundary value. This detection could also be called xe2x80x9cmeasurement in a broader sensexe2x80x9d as well.
In the pneumatic length measurement under consideration there are various circuits, especially a circuit xe2x80x9cnetwork with bridge.xe2x80x9d If the aforementioned pressure measurement process is accomplished with the circuit xe2x80x9cnetwork with bridgexe2x80x9d and a difference pressure measurement device is used as the measurement device (compare DIN 2271, Part 1, Subsection 6.2.2), the difference between the pressure in the measurement branch, i.e., pressure between a preliminary measurement nozzle and the measurement nozzle, and the pressure in the reference branch, i.e., pressure between the preliminary reference nozzle and the reference nozzle, is detected as the difference pressure. Under the assumptions that the measurement nozzle and the reference nozzle are identical in terms of flow engineering, therefore having the same outlet openings, and that the reference surface assigned to the measurement nozzle and the reference surface assigned to the reference nozzle are identical in terms of flow engineering, at the same distance between the measurement nozzle and the reference surface on the one hand and between the reference nozzle and the reference surface on the other the difference pressure is zero.
The above explained pressure measurement process in a network with a bridge has especially two advantages. On the one hand, the difference between the measurement nozzle-reference surface distance and the reference nozzle-reference surface distance can be detected especially well. On the other hand, changes of the feed pressure, in any case within certain limits, remain without any influence on the measurement result, therefore, on the determination of the distance between the measurement nozzle and the reference surface.
A measurement apparatus which works using the pressure measurement process and in which there is a network with a bridge is always treated below. Likewise all other embodiments will also be encompassed, therefore especially also the flow rate measurement process and the speed measurement process.
The possible applications of pneumatic length measurement to which the invention relates are diverse (compare DIN 2271; Part 4, Section 3 xe2x80x9cInstructions for usexe2x80x9d and Section 4 xe2x80x9cApplication examples,xe2x80x9d the literature citation xe2x80x9cPRODUCTION ENGINEERING 1xe2x80x9d loc. cit., page 50, and the brochure xe2x80x9cSYSTEM FOR PNEUMATIC-ELECTRICAL CONTROL AND MEASUREMENT,xe2x80x9d loc. cit., pages 7 and 20). Especially important possible applications of pneumatic length measurement are proximity monitoring and seating monitoring. For seating monitoring on a clamping device (some, production machine or machine tool) there is the problem of monitoring proximity or correct seating of the workpiece in the clamping means; if the seating of the workpiece at a certain point is no long ensured, the clamping device will not be able to be activated.
Since conventional measurement apparatus cannot be used as comprehensively as is desired by the user or potential user, the object of the invention is to embody and develop the measurement apparatus such that within wide ranges, i.e., for as many applications as possible, it can be easily and especially simply used.
The measurement apparatus in accordance with the invention is characterized in that the effective outlet surface (annular gap surface) of the reference nozzle can be adjusted in increments or continuously. (For what is meant by effective outlet surface, annular gap surface of the reference nozzle, reference is made to DIN 2271, Part 1, Section 2 xe2x80x9cPhysical principlexe2x80x9d). Because in the measurement apparatus in accordance with the invention, the effective outlet surface (annular gap surface) of the reference nozzle can be adjusted in increments or continuously, both a xe2x80x9cmeasurement in the broader sensexe2x80x9d and a xe2x80x9cmeasurement in the narrower sensexe2x80x9d can be taken. Therefore, the measurement apparatus allows both information about whether the distance of the object to the reference surface is greater than or less than a given boundary value (or corresponds exactly to the boundary value) and also measurement of this distance without reference to a boundary value, therefore a xe2x80x9cmeasurement in the narrower sense.xe2x80x9d
As previously mentioned, it can be necessary for either the compressed air source to make available air with a desired feed pressure, relatively accurately and constantly over time, or a pressure regulator can be provided for making available air with a certain feed pressure relatively constantly over time.
In accordance with the invention, the necessity of making available air with a certain feed pressure relatively constantly over time can be qualified if the feed pressure of the compressed air made available by the compressed air source is measured and delivered to the measurement device as a reference, control and/or correction value.
To date it has always been assumed that the measurement device acquires one state variable or the measurement devices acquire at least one of the three state variables, flow rate, pressure and speed of the air upstream of the measurement nozzle or measurement nozzles. According to another teaching of the invention, it can be provided that the measurement device does not acquire one state variable, or the measurement devices do not acquire at least one of the three state variables, flow rate, pressure and speed of the air upstream of the measurement nozzle or measurement nozzles, but rather the time change of one or at least one of the three state variables flow rate, pressure and speed of the air upstream of the measurement nozzle or measurement nozzles. If one or at least one of the three state variables, flow rate, pressure and speed of the air upstream of the measurement nozzle or measurement nozzles is detected, it can take a relatively long time, roughly 400 ms to 2000 ms before a stable end value is reached, specifically due to the compressibility of air and especially when the volume of the system is relatively large. If the time change of one or at least one of the three state variables flow rate, pressure and speed of the air upstream of the measurement nozzle or measurement nozzles is detected, the required measurement time can be greatly reduced, for example, to roughly 50 ms.
Otherwise, it has always been assumed so far that the invention relates to pneumatic length measurement or pneumatic distance measurement or determination, therefore, air is used. Instead of using air, however, liquids such as cooling or lubricating liquids an also be used. In this regard, instead of pneumatic operation, hydraulic operation is also possible.
In particular there are various possibilities for embodying and developing the measurement apparatus and process of the present invention.